Costa's relationship with Blues boss Antonio Conte broke down following a row in January over a potential move to Chinese Super League club Tianjin Quanjian.
Last month, Conte indicated the incident had made up his mind over the future of Costa – who has been linked with Atletico Madrid and AC Milan – and the arrival of Alvaro Morata from Real Madrid has bolstered Chelsea's options up front.
However, Chelsea have been left perplexed by claims made by Costa's representative Ricardo Cardoso, who threatened to take legal action against the club having "dismissed" him by text message.
Conte, who was asked about the rumours when addressing the media ahead of Chelsea's Community Shield clash with Arsenal, is reported to have told Costa's agent Jorge Mendes in January that the he was free to leave during the close-season.
Chelsea's director of communications Steve Atkins interjected the questions to say "the premise the lawyer has put forward is wrong", labelling the suggestions as "just nonsense".
And Conte is adamant the situation has not changed, saying: "For me it's very simple the situation, and I repeat what I said in the past, maybe 10 days ago.
"The player, his agent and the club knew very well the position of the player in the summer.
"I don't understand why the lawyer gets involved. I don't understand this, but the situation is very clear and always the same.
"For me this is the past."